Color printers are becoming more popular and are taking over from black and white printers such as monochrome inkjet and dot matrix printers. Moreover, the color quality of the prints made by color printers has become photo-like due to the increase of the number of inks used from the conventional 3 different color inks to as many as 6 or 8 different color inks, thereby increasing the color gamut of the printers allowing for closer reproduction by the printers of the colors of the original image and monitor.
In a traditional color printing system, color image data composed of three-dimensional color signals supplied to a personal computer from a color image scanner is displayed on a color monitor and also printed by a color printer.
Additionally, traditional color printers are based on 4-color printing, using black (K), in addition to the three primary colors of cyan (C), magenta (M), and yellow (Y). Theoretically black can be produced by mixing the three CMY colors; however, due to the difficulty in achieving pure black due to impurities in the ink, it's common to add black as a fourth color for printing.
Currently six-and seven-color printers are also available, in which light cyan, light magenta, and other colors are added to the CMYK primaries.
An image displayed on a monitor using the three RGB primary colors must be converted to CMYK for printing. Each computer printer comes with printer driver software that converts color images created on the computer into a data format that can be processed by the color printer.
Monitors and scanners that use the three RGB primary colors, and color printers and printed matter that use the three CMY primary colors, each have a different range of reproducible colors. The full range of colors that can be produced by any color reproduction system is called the color “gamut” of that system. Thus, the monitors, scanners and color printers have different color gamuts.
A picture of all available colors (a color “space”) is often drawn as a colored disk. The colored disk is typically a “plane” of a “CIE color space”. The color gamuts of individual devices are then drawn on the available gamut as polygons. For color monitors, printers and scanners the polygons typically have six sides corresponding to the six “primary” colors: cyan, magenta, yellow, red, green, and blue. The area inside a polygon represents all the colors that can be achieved with that particular device.
If the color gamut of a color printer and a color monitor are both drawn on the colored disk then the color printer gamut will typically fall within (form a subset of) the color monitor gamut. This is because the gamut of colors that can be reproduced by a CMYK color printer is smaller than what can be shown on an RGB monitor. Thus, the full range of colors that can be displayed on the color monitor cannot be reproduced by the color printer. As a result, RGB colors that look wonderful on a computer screen sometimes become dull or less saturated when converted to CMY (or CMYK) for a color printout.
Gamut mapping is a technique for adjusting the color across different devices so that the image seen by the human viewer will be as consistent as possible when reproduced on devices with different ranges of reproducible color. This technique is used by color management systems (CMS).
There are several different methods for gamut mapping. One simple solution is to move all the points of the color monitor polygon directly inward to the nearest point on that color printer polygon, while matching all other points as accurately as possible. This provides the best possible match to all colors that can be accurately matched, and is great for hitting spot colors, but it tends to produce lousy reproductions of photographs.
Consider a photograph of an apple in which the reds of the highlights have to all be moved, and that by these rules they are all moved to the same point on the color printer polygon. As we view the photograph, we'll see a terrible “fringe” surrounding the highlight as the area of out-of-gamut colors that have been run-together transitions to the area where more accurate color reproduction is possible.
This is often called a “colorimetric” correction which results from a “colorimetric ICC profile”.
A more satisfactory solution is to “deform” the entire surface of the color monitor gamut so that all points are moved into the color printer polygon, while avoiding “clipping” colors so that colors that differed in the original are knocked down to be the same color in the reproduction. Colors that are within both of the gamut polygons will be less accurately reproduced, but the reproductions will be free of the “fringes” described above. This is often called a “perceptual” or “photometric” correction which results from a “perceptual ICC profile”.
The problem with the color printing process remains that computer monitors have larger color gamuts than do the color printers to which they print. While the existing gamut mapping is helpful, “what you see” on the monitor is not “what you get” from the color printer. It would be desirable to be able to view a preview of an image on a computer monitor that would have the same color quality as what would be printed.